In addition to the title discussion, this post will serve as the Mid-July 2012 Sea Surface Temperature Anomaly Update. It also includes a status update on my book about El Niño-Southern Oscillation (ENSO).
COMPARISON OF THE EVOLUTIONS OF EL NIÑO EVENTS
NINO3.4 sea surface temperature anomalies (a commonly used ENSO index) have been above the +0.5 deg C threshold of an El Niño for 4 weeks. While it’s far from an “official” El Niño, it appears that it’s likely to become one. Let’s see how the 2012/13 El Niño is evolving compared to past El Niño events. Figure 1 compares the weekly NINO3.4 sea surface temperature anomalies for each El Niño event since 1982, starting with the first week in January of those years. The 2012 data is in red, using a greater weighting. The first thing that stands out in the graph is how there really is nothing typical about the evolution of El Niño events. Five started from ENSO-neutral conditions; that is, with NINO3.4 sea surface temperature anomalies between -0.5 and +0.5 deg C. Five, including the current one, started from La Niña conditions, with the NINO3.4 sea surface temperatures cooler than -0.5 deg C. And there’s the outlier, the 1987/88 portion of the 2-year 1986/87/88 El Niño. Other than having the coolest NINO3.4 sea surface temperature anomalies at one point, there’s nothing remarkable about the evolution of the NINO3.4 sea surface temperature anomalies this year.
Figure 2 compares the evolution of the El Niño events that started from La Niña conditions. This year’s NINO3.4 sea surface temperature anomalies had been tracking along at the pace of the most recent El Niño, the one that occurred in 2009/10, until recently. Over the past two weeks, NINO3.4 sea surface temperature anomalies have been cooling.
NINO3.4 sea surface temperature anomalies appear as though they’re being suppressed by the cooler-than-normal waters being circulated southward from the North Pacific, which should be feedback from the back-to-back La Niña events. Refer to the sea surface temperature animation from Unisys, Animation 1, but keep in mind that positive temperature anomalies are light blue. Most people associate shades of blue with negative anomalies. (You may need to click on the animation to start it.)
It will be interesting to see how long the cooler waters from the North Pacific can suppress the central sea surface temperatures in the east-central equatorial Pacific.
Figure 3 shows the NINO1+2 sea surface temperature anomalies for the same El Niño events that were presented in Figure 2. The NINO1+2 region is in the eastern tropical Pacific, just south of the equator. The coordinates are 10S-0, 90W-80W. This year the NINO1+2 sea surface temperature anomalies warmed before the NINO3.4 region, but they also have been cooling.
But referring to the animation of NOAA subsurface temperature anomaly cross sections for the equatorial Pacific, Animation 2, there’s still a pocket of elevated anomalies at depth in the eastern equatorial Pacific, and there’s a long way to go before the peak of the ENSO season.
Weekly NINO3.4 sea surface temperature anomalies for the week centered on July 11, 2012 are approximately +0.55 deg C, having dropped from about +0.73 over the past few weeks.
And global sea surface temperature anomalies are continuing the upward march, rebounding from La Niña conditions and responding to the evolving El Niño.